This invention relates to a valve for an anaesthetic rebreathing system.
The terms "anaesthetic system" and "anaesthetic rebreathing system" are used, in this specification, to denote the connecting apparatus between the anaesthetic machine and the face mask or endotracheal tube which connects to a patient, except where the context indicates otherwise.
A simple anaesthetic breathing system comprises two lengths of flexible tubing of suitable dimensions which connect the patient's airway to an anaesthetic gas supply machine. The two lengths of flexible tubing constitute the inspiratory and the expiratory limbs of a breathing circuit and converge at the patient's airway so that fresh gas flows from the anaesthetic gas supply machine, through the inspiratory limb of the circuit, to the patient's airway. Exhaled gas flows out through the expiratory limb of the circuit and is vented at the end of the expiratory limb. In a simple anaesthetic breathing system therefore, the flow of gas remains unidirectional.
Anaesthetic rebreathing systems have a compliant reservoir bag placed in the circuit for intermittent reversal of gas flow in whichever of the inspiratory or expiratory limbs the reservoir bag is placed. In a Mapleson A, system the inspiratory limb contains the reservoir bag and this configuration is suitable when a patient breathes spontaneously. In a Mapleson D system, the expiratory limb contains the reservoir bag and this configuration is suitable when the breathing of a patient is mechanically or manually controlled, for example by compression of the reservoir bag. Such compression of the reservoir bag results in the rebreathing of exhaled gases which inter alia provides useful humidification in the patient's airway.
A Mapleson E system, like a Mapleson D system, is also suitable when the breathing of a patient is manually or mechanically controlled. However, in a Mapleson E system, the reservoir bag is either open-tailed or is replaced by a mechanical ventilator. In this system the excess gas is vented through a valve or opening external to the breathing circuit itself. A Mapleson D system can easily be converted to a Mapleson E system by closing a relief valve which is in the circuit and by replacing the closed reservoir bag in the expiratory limb with a mechanical ventilator or open-tailed reservoir bag.
It is important that in a particular situation the mode of breathing namely spontaneous breathing or controlled breathing, is appropriate. If the inappropriate mode of breathing is used one or more problems can arise. For example, at least twice the fresh gas flow may be required to prevent retention of carbon dioxide produced by the patient which would be wasteful.
Valve-controlled, switchable rebreathing circuits are known which allow for the selection of a particular mode of breathing. However, an inherent danger of such circuits is that accidental, incorrect use of the valve could have disastrous consequences. For instance, if an open-tailed reservoir bag or a mechanical ventilator was attached to a valve-controlled, switchable circuit in place of the closed reservoir bag, and the selector member was inadvertently moved so that the system was converted to a Mapleson A system, there would be the following undesirable consequences:
a) the patient would be breathing an excessive and dangerous amount of spent gas; and PA1 b) the fresh gas entering the inspiratory limb would simply be vented through the reservoir bag or the mechanical ventilator and would be wasted.